Electric power distribution networks are widespread and are capillary like. These networks primarily transport and distribute electric energy, but the possibility of exploiting it also as a medium for data transmission is well known. Electrical mains can be advantageously used to establish communication with far away locations exploiting the fact that even remote rural users are reached by an electrical power line. It is also possible to avoid the cost of a dedicated line (e.g., a telephone line) for telecommunications.
Data transmission on electrical mains is possible since AC power is distributed with a well determined frequency. Signal modulation techniques permit the transmission of information on or about a certain carrier frequency that may advantageously be in a frequency band not occupied by other signals. It is possible to transmit on the same line several signals, each occupying a different frequency band. The receiver selects the desired signal by tuning to the relative carrier frequency.
Data transmission on power lines may be a particularly efficient technique for controlling machines installed in a remote location, without being forced to install a dedicated telecommunication line. Because of the advantages provided by such a data transmission technique, transceivers for coupling to power lines have an increasing importance.
A telecommunication station coupled to power lines generally includes a microprocessor interfaced with a transceiver coupled to the power line. Power line transceivers are well known and commercially available. Examples of effective transceivers are described in U.S. Pat. No. 4,714,912 and U.S. Pat. No. 5,842,032.
Generally, known transceivers do not directly interface with a microprocessor. Moreover, these transceivers require dedicated interface devices for coupling with the electrical mains to meet the requirements of the communication standards on the network, establishing access criterions, avoiding frequency bands reserved to electricity producers and frequency bands reserved for home applications. It would be desirable to have a fully integrated transceiver allowing the realization of a telecommunication station with the above mentioned characteristics, capable of supporting the remote managing of electrical loads connected to the electrical mains.
Often the conditions for using such a telecommunication system on electrical power lines are those of communicating with people at their homes, such as in applications generally referred as home applications. In these application areas, the cost of apparatuses has a great importance.
Usually a digital data transceiving station specifically uses a multichannel transceiver coupled through an appropriate interface to an electrical power distribution network. Also, the digital data transceiving station may generally comprise a modem interfaced with a microprocessor by way of a specific communication circuit between the modem and the microprocessor, commonly called a serial interface.
In known systems, the possible choices are either to establish a Bit Mode or a Packet Mode communication between the modem and the microprocessor through the serial interface. Binary serial transmission between the modem and the microprocessor can be made in two different ways. The first is the data transceiving station, where each time the modem has a demodulated bit, it transmits it to the microprocessor. The second is the Packet Mode. In this mode the modem stores a pre-established number of bits forming a packet of bits that eventually is transmitted to the microprocessor.
Bit Mode communication between the modem and the microprocessor does not introduce any data format because bits are transmitted immediately after the modem has decoded them. It is easy to understand that the Bit Mode has the advantage of being usable irrespective of any particular data format, but its drawback is that the rate of communication between the modem and the microprocessor is limited to that of the communication channel.
In contrast, in Packet Mode communication the rate of communication between the modem and the microprocessor can be greater than that over the channel. However, it is not independent of the particular data format. Bit Mode communication ensures compatibility of the system irrespective of the data format used, but this approach imposes the use of a microprocessor having adequate computing capacities to interpret the bit stream received by the serial interface for its information content.
The technical alternative of establishing a Packet Mode communication between the modem and the microprocessor is advantageous because it ensures a faster communication and allows the use of a relatively low cost microprocessor for the same global performances of the station. However, it is usable only by operating with a certain predefined data format.
For example, if the protocol format has data in frames being transmitted with a preamble followed by a header and by a data field alternated with synchronization signals, formatting the preamble into packets would destroy its information content. If data were formatted in M bit words and the Packet Mode transmission forms packets of N bits, the microprocessor would be forced to process the received data to extract the original M bit words. This would waste the benefits of the greater communication speed that may be achieved with a Packet Mode transmission.
Therefore, there is a clear need and/or utility of having a digital data transceiver wherein communication between the modem and the microprocessor through the serial interface may switch back and forth between a Bit Mode and a Packet Mode during the time slot in which a single data frame is transmitted. This switching is done without any loss of data.